The present invention relates to the manufacture of semiconductor-on-insulator (SOI) structures using an improved process for making same.
To date, the semiconductor material most commonly used in semiconductor-on-insulator structures has been silicon. Such structures have been referred to in the literature as silicon-on-insulator structures and the abbreviation “SOI” has been applied to such structures. SOI technology is becoming increasingly important for high performance thin film transistors, solar cells, and displays, such as active matrix displays. SOI structures may include a thin layer of substantially single crystal silicon on an insulating material.
Various ways of obtaining SOI structures include epitaxial growth of semiconductor material on lattice matched substrates. An alternative process includes the bonding of a single crystal silicon wafer to another silicon wafer on which an oxide layer of SiO2 has been grown, followed by polishing or etching of the top wafer down to a thin layer of single crystal semiconductor. A further method includes ion-implantation, where either hydrogen or oxygen ions are implanted to form a weakened layer in a donor semiconductor wafer in order to separate (exfoliate) a thin semiconductor layer from the donor wafer and bond same to the insulator substrate.
Although many SOI manufacturing processes call for round single crystal (e.g., monocrystalline) semiconductor layers atop an insulator substrate, there are a number of applications for which non-circular semiconductor layers are required. For example, the provision of rectangular semiconductor layers (sometimes called “tiles”) is becoming a requirement in the manufacture of displays, including organic light-emitting diode (OLED) displays and liquid crystal displays (LCDs), integrated circuits, photovoltaic devices, thin film transistor applications, etc.
The rectangular semiconductor layers must often be manufactured using very tight tolerances, crystal orientation accuracy, and strict form accuracy, including edge straightness, parallelism, and perpendicularity. The semiconductor layers should be free of contamination, mechanical damage (including subsurface damage), etc., which can erode or even catastrophically degrade the performance of a completed SOI structure. When rectangular semiconductor layers are formed using the aforementioned exfoliation/bonding technique, then it is desirable for the donor semiconductor wafer to be of a generally right parallelepiped configuration. Such a donor semiconductor wafer should adhere to stringent specifications, including having all four corners of at least one surface rounded, and all four straight edges of such end profiled (e.g., rounded in the manner of chamfering). These tight form requirements are desirable so that the donor semiconductor wafer can endure the processing stresses imposed by a significant number of re-uses (to produce many exfoliation layers and, thus, many SOI structures).
Rectangular donor semiconductor wafers may be produced from dedicated ingots or they may be machined from cylindrical semiconductor wafer stock (so-called round wafer stock). The commercial viability of the former is constrained by the availability of semiconductor ingots in a right parallelepiped configuration. Many semiconductor wafer manufacturers are unwilling to dedicate manufacturing resources to the production of rectangular semiconductor ingots because the market demand for such ingots is presently very small. The commercial viability of the latter approach (using round wafer stock) is more attractive at present because there is significant availability of 300 mm diameter round wafer stock.
The use of round wafer stock to produce rectangular semiconductor layers for use in SOI structures requires that the round wafer stock be processed into rectangular donor semiconductor wafers. A variety of methods have been employed to machine rectangular donor semiconductor wafers out of round stock. Diamond dicing round stock into a rectangular structure has been the traditional process due to its ability to hold strict form accuracy. Diamond dicing processing, however, leaves micro-cracks and chips on the straight edges and corners of the resultant rectangular donor wafer. In addition, the diamond dicing process does not readily produce rounded corners in the rectangular donor wafer. This severely limits the viability of re-use of the donor wafer. Indeed, as the donor semiconductor wafer is exposed to significant thermal and mechanical stress during the formation of an SOI structure, the subsurface damage created by diamond dicing would likely lead to chipping or breakage of the donor wafer when exposed to multiple re-uses.
Mechanical dicing a round stock ingot into a rectangular structure followed by extensive edge forming (edge grinding and polishing) has also been employed. While effective, such mechanical dicing and post processing add considerable fixed and variable costs to the overall process of producing SOIs, which limits the commercial viability of the technique. Indeed, edge grinding and polishing technology, and related equipment, are prohibitively expensive and the associated consumables needed to carry out such edge treatments must be purchased and managed, often requiring additional expensive preparation and control systems. Further, extensive edge forming processes reduce the usable surface area of the resultant semiconductor layer of the SOI. Finally, such edge forming processes often only conceal subsurface damage and, thus, fracture strength remains compromised.
Although the manufacturing processes for making SOI structures are maturing, the cost of producing such structures and the final products employing them is driven in part by the ability to obtain suitable rectangular donor semiconductor wafers at commercially viable prices. Accordingly, it is desirable to continue to advance the technologies associated with producing donor semiconductor wafers in order to control the cost of manufacturing SOI structures.